The hybrid vehicle generally has two different power sources simultaneously. According to the various connection types, it may be mainly divided into three structures of series hybrid, parallel hybrid and series-parallel hybrid.
The series hybrid is characterized in electrically coupled, wherein the engine directly drives an electric generator for charging the battery, and then an electric motor is operated by electrical energy from the battery to drive the wheels. The parallel hybrid is characterized in mechanically coupled, wherein the engine is connected with the driving shaft, and the motor can be selectively operated as an electric motor or a electric generator for balancing the loads of the engine.
In the prior art, one of the series-parallel hybrid may use a planetary gear unit to realize the distribution of power among the driving units. In this structure, the engine may be connected to the carrier of the planetary gear unit, a part of the power of the engine is transferred into the electrical energy by means of the first motor for furnishing the electrical energy to the second motor or for charging the battery, and the other part of the power of the engine is directly transmitted to the ring gear of the planetary gear unit; meanwhile, the second motor is connected with the ring gear for the supply of some supplementary power or torque. In this structure, a part of the power output from the engine is always used to power the first motor to generate the electrical energy for charging the battery or for driving the second motor.
In the prior art, a series-parallel hybrid may use the clutch to realize the power distribution among the driving units. For example, the Chinese patent CN 1200824 C discloses a hybrid power drive system for propelling the vehicle, which comprises two clutches, two motors and a battery, wherein the first clutch and the second clutch are respectively disposed between the first motor, the second motor and the internal combustion engine, the second motor is permanently connected to a variable-ratio transmission, and the first motor and the second motor are connected electrically with the battery. As it can be seen from the disclosure, the first motor of the drive system has three functions as below: (1) starting motor for the internal combustion engine; (2) generator for outputting electrical energy; and (3) drive device for driving the transmission hydraulic pump and other auxiliaries such as an air conditioner compressor, a power steering pump. Thus it can be seen that, the first motor is equal to a starting motor (with lower power or capacity) for the internal combustion engine in a conventional vehicle, except that it is electrically connected with the battery, and under control, it has some other functions besides to start the internal combustion engine, for example, to drive some auxiliaries. The Chinese patent CN 1200824 C also discloses that, during the vehicle is propelled by the second motor, when a greater traction torque is needed, the engine can be started by the first motor independent from the second motor, and then the engine and the second motor together propel the vehicle. In this case, the structure reduces, at some extent, the response time for vehicle acceleration. However, due to the limitation in function and design of the first motor, the first motor can't be operated for driving the wheels alone, and unsuitable for driving the wheels together with the second motor. Therefore, when a rapid acceleration from rest is required for the vehicle, the second motor will be operated for propelling the vehicle alone before the engine is started. Because the power of the second motor is relatively small, the acceleration property is limited during this period, and the acceleration time will be longer. Furthermore, the drive system can't provide more driving modes, and can't provide optimum power output under various driving conditions.